To make an Item Billable, or to make an Item Non-Billable, edit that Item's status in the "Billable" column of your Items (this can be done on either the Project Details Items list, or the Items List page):
A green checkmark indicates that the Item is Billable, a grey line indicates that the Item is Non-Billable. To change this status for an Item, click on the checkmark or line you wish to change the status of, then select the appropriate option.
Changing an Item to "Billable" provides you with options for how that Item bills, whether it is an Item type which bills against time:
Or an Item type which bills against units:
Select the desired method of billing to change to Billable, or select "Not Billable" to change to Non-Billable.